<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        * {
            margin: 0;
            padding: 0;
        }
        #up,#down {
            height: 200px;
            width: 200px;
            background: pink;
            margin: 50px;
        }
    </style>
</head>
<body>
    <div id="up"></div>
<!--    让up和down不在同一bfc下-->
    <div style="overflow: hidden">
        <div id="down"></div>
    </div>
    <!--<div id="up"></div>
    &lt;!&ndash; br占21px &ndash;&gt;
    &lt;!&ndash;<br/>&ndash;&gt;
&lt;!&ndash;    up 和down不相邻&ndash;&gt;
&lt;!&ndash;    <div style="height: 1px"></div>&ndash;&gt;
    <div id="down"></div>-->
</body>
</html>
